In the manufacture of the arc tube for a high pressure sodium discharge lamp it is necessary that the tubulation, which extends through at least one of the end caps to permit the evacuation and filling of the arc tube with the discharge sustaining medium, be effectively hermetically sealed off to complete the arc tube manufacturing process.
An effective and commercially feasible method for sealing off the refractory metal tubulation of a high pressure sodium discharge lamp arc tube is disclosed in U.S. Pat. No. 3,566,067 to Daniel A. Larson in which two sets of pinching jaws are employed, the first to flatten a section of the tubulation and the second to provide a narrowly pinched section which will serve as a point of separation when a predetermined electrical current is passed through the tubulation. Although this method of sealing off the refractory metal exhaust tubulation of a ceramic arc tube has proved reasonably successful, in some instances because of the very thin section of tubulation at the point of separation and the current levels necessary to cause the melting and separation, some of the metal in this thin area is flashed away resulting, in some instances, in unreliable seals.
An improvement on this method of tipping off the refractory metal tubulation of the high pressure sodium discharge lamp arc tube is disclosed in U.S. Pat. No. 4,117,299, issued to John Petro et al., which discloses a single four-directional crimp of the tubulation to form an X-shaped cross section. This latter method provided more metal in the area of the electrical resistance separation and provided an improved seal.
Although the foregoing methods reduced the tube diameter at the parting location significantly, the use of a single force to move the metal to a different shape and size provided some weakening and distortion of the metal structure, causing fine cracks both along the melted edge as well as in the remainder of the heavily squeezed area and in some instances has been the source of lamp failures.